I've been practicing Krishna consciousness for three months - trying to chant sixteen rounds and follow all the principles - but I recently had illicit sex with my wife and I feel terrible.

You're a young devotee and seem to be sincere in your desire to advance in Krishna consciousness. Even in Bhagavad-gita Lord Krishna says that if one is sincerely trying to become a devotee but falls down due to past bad habits he is still to be considered a devotee.

You shouldn't be too hard on yourself, especially this early in your Krishna Consciousness. There are many unwanted things in the heart that need to be cleared over time. It will take patience and chanting over time, also service. So please just be patient and continue with your chanting and reading.